It is a well known fact that lithium-ion secondary batteries, which have high energy density and show no significant decrease in discharge capacity, have been used to power sources for mobile tools, such as mobile phones and laptop computers. In recent years, with downsizing mobile tools, there is a need for smaller lithium-ion secondary batteries for use therein. Further, with the development of hybrid cars, solar power generation, and other technologies from the viewpoint of preventing global warming, etc., the application of super capacitors having high energy density, such as electrical double-layer capacitors, redox capacitors, and lithium ion capacitors, has been expanding at an accelerated pace, and there is a need for a further increase in energy density.
An electrical storage device, such as a lithium-ion secondary battery or a super capacitor, is configured, for example, such that a positive electrode, a negative electrode, and a separator made of polyolefine or the like between them are arranged in an organic electrolytic solution containing as an electrolyte a fluorine-containing compound, such as LiPF6 or NR4.BF4 (R is an alkyl group). The positive electrode comprises a positive electrode active material, such as LiCoO2 (lithium cobalt oxide) or active carbon, and a positive electrode current collector, while the negative electrode comprises a negative electrode active material, such as graphite or active carbon, and a negative electrode current collector. With respect to their shape, generally, a current collector having applied thereon an active material is formed into a sheet. The electrodes are each subjected to a large voltage and also immersed in a highly corrosive organic electrolytic solution that contains a fluorine-containing compound. Accordingly, materials for a positive electrode current collector, in particular, are required to have excellent electrical conductivity together with excellent corrosion resistance. Under such circumstances, currently, nearly 100% of the time, the material for a positive electrode current collector is aluminum, which is a good electrical conductor and also forms a passive film on the surface to offer excellent corrosion resistance (as materials for a negative electrode current collector, copper, nickel, and the like can be mentioned).
One method for providing an electrical storage device with downsizing and higher energy density is thinning a sheet-shaped current collector that forms an electrode. Currently, an aluminum foil having a thickness of about 15 to 20 μm produced by rolling is commonly used as a positive electrode current collector. Therefore, the object can be achieved by further reducing the thickness of such an aluminum foil. However, in rolling, it is difficult to further reduce foil thickness on an industrial production scale.
Then, a possible aluminum foil production method to replace rolling is a method for producing an aluminum foil by electrolysis. The production of a metal foil by electrolysis is performed, for example, by forming a metal film on a surface of a substrate such as a stainless steel plate by electroplating, followed by the removal of the film from the substrate. Such production is well known as a method for producing a copper foil, for example. However, aluminum is an electrochemically base metal, and thus electroplating is extremely difficult. Therefore, it is not easy to produce an aluminum foil by electrolysis. Patent Document 1 discloses, as a method for producing an aluminum foil by electrolysis, a method that uses an electrolytic bath containing 50 to 75 mol % aluminum chloride and 25 to 50 mol % an alkylpyridinium chloride or an electrolytic bath prepared by adding an organic solvent to such a bath. However, in this method, the chlorine concentration in a plating solution is extremely high. This leads to problems in that during plating, chlorine contained in the plating solution reacts with moisture in the air to generate hydrogen chloride gas, causing the corrosion of equipment. Therefore, it is necessary to take a measure to prevent the generation of hydrogen chloride gas or a measure to protect equipment from corrosion due to the generated hydrogen chloride gas. Further, the method of Patent Document 1 also has problems in that the applicable current density is about 2 A/dm2 maximum, and thus the film formation rate is low (when the applied current density is increased any further, the plating solution decomposes, etc., making it impossible to stably perform plating). The addition of an organic solvent, such as benzene or toluene, to the plating solution is expected to improve the film formation rate. However, these organic solvents have high toxicity and are dangerous because of high inflammability, and, therefore, it must be said that the addition of such organic solvents to a plating solution is problematic in terms of the ease of liquid waste disposal and safety.